With populations increasing and newly developed communities springing forth in previously uninhabited locales, the demand for power is testing the capacities of energy providers. New sources of fuel or power are being sought daily and sources previously considered inefficient to tap are being studied further to extract whatever power generation may be available. However, fossil fuels such as coal and petroleum, previously favored because of their abundance and inexpense are now becoming shunned because of the presumed deleterious effects their consumption has effected on the environment and their increasing costs. Additionally, there are some that feel the United States has become dependent on the production of fuel from foreign nations and thus, we should seek alternatives to these fuel sources. In response, industries are converting more machinery previously powered by petroleum-based products to electric-based power sources. Additionally, the population as a whole is becoming increasingly dependent on electrical equipment to manage our businesses, transport our workforce, and run our homes. Thus, the search for alternative energy sources is gathering increasing interest with the hope that natural power sources will provide enough clean energy to power our nation's increasing energy demands. One source expected to help meet the need for meeting the for energy production is wind power.
Wind power is the conversion of wind energy into more useful forms such as electricity. Wind energy is considered by many an ample, renewable, widely distributed and clean power source that mitigates the greenhouse effect if used to replace fossil-fuel-derived electricity. Wind power is for the most part relegated to large scale wind farms for use in national electrical grids. Small individual turbines are used for providing electricity to rural residences or grid-isolated locations because of the current structural capabilities and the economic obstacles associated with generator manufacture and territorial placement.
Most major forms of electric generation are capital intensive, meaning they require substantial investments at project inception but have low ongoing costs (generally for fuel and maintenance). This is particularly true for wind power which has minimal fuel costs and relatively low maintenance costs. However, wind power has a high proportion of up-front costs. The “cost” of wind energy per unit of production is generally based on average cost per unit, which incorporates the cost of construction (including material components), borrowed funds, return to investors, estimated annual production, among other components. These costs are averaged over the projected useful life of the equipment, which can be in excess of twenty years if the generator equipment maintains durability and efficient production. Thus, minimizing the risk of premature breakdown while extracting the most power from a given locale becomes a compelling goal when fabricating a wind power generation source. One of the most common and widely used structures for wind power extraction is a wind turbine.
A wind turbine is a machine the converts kinetic energy from the wind either into mechanical energy used directly by machinery such as a pump or is then converted into electricity which is subsequently used to power electric equipment. Wind turbines are popular sources of power because they do not rely on the burning of fossil fuels whose consumption is a known contributor to the pollution of the environment. Wind turbines are commonly separated into two types: horizontal axis wind turbines or vertical axis wind turbines. For this application, discussion will focus on a wind turbine blade for use on a horizontal axis wind turbine. Such wind turbines have a main rotor shaft and electrical generator at the top of a tower and are pointed into the wind. Common modern wind turbines are pointed into the wind and controlled by computer-controlled motors. The blades should be made stiff and strong to resist bending, shear, and torsional forces produced by strong winds. Horizontal axis wind turbines are popular amongst energy harvesters because the design of the blades and their placement are conducive to self starting and operation whenever the blades are subjected to winds.
In practice, wind generators are usually sited where the average wind speed is 10 mph or greater. An “ideal” location would have a near constant flow of non-turbulent wind throughout the year and would not suffer from excessive sudden, powerful wind gusts. Current preferred sites include windy areas such as hilly ridgelines, shorelines, and off-shore developed platforms situated in shallow waters. However, an important turbine siting consideration is access to or proximity to local demand or transmission capacity and such typical sites are distant from local demands; especially those growing demands created by burgeoning communities in flat, low wind-speed areas. Low wind-speed areas have wind power potential, however, the current technology is considered by some inefficient and/or cost prohibitive for use near to these locales.
During the general operation of a wind turbine, the air that passes over the upper camber of an airfoil must travel faster than the air traveling under the lower camber. Thus, a difference in pressure is formed where the air traveling over the upper camber is at a lower pressure than the air traveling under the lower camber. This results in a lift force on the blade, which induces a torque about the rotor axis, causing the turbine to rotate. Thus, energy is extracted from this torque on the wind turbine blades.
Several factors contribute to the efficiency of a wind turbine system. The one important factor is the length of the blades, as the total power that can be extracted is proportional to the disk area swept by the rotor blades as they rotate, which is proportional to the square of the blade length. Other factors include the ability of the control system to maintain the optimal tip speed ratio. Factors such as low blade weight and low rotational inertia of the rotor make it easier for the control system to maintain the ratio between wind speed and blade rotation speed, increasing and decreasing the rotor speed as wind speeds fluctuate.
One obstacle to the development of longer wind turbine blades necessary to increase the disk area and power production is the rapid increase in blade weight as the blade length increases. As blade length increases, the loads on the blade increase rapidly. A longer blade is also more flexible than a shorter blade. In order to resist the increased loads and to provide the required stiffness, a significant amount of additional material must be added to longer blades to maintain structural integrity. The addition of material increases blade material cost, as additional material must be purchased and processed. Additional weight in a wind turbine blade is detrimental because it increases certain loads on the hub and generator systems due to the increased rotational inertia of the rotor disk, and the increased gravity loads on the blades. Furthermore, additional weight can be detrimental because it can cause a reduction in the natural vibration frequencies of the blades, potentially causing undesirable interactions with the airstream and/or the dynamics of the tower and support structure.
Therefore, any method making more efficient blade structure has the potential to reduce the material cost, and to allow larger blades to be built. In some cases, these larger blades may be combined with existing generators, allowing additional power to be generated, especially in low wind speed areas. This is important, as a large fraction of the United States has relatively low wind speeds. Furthermore, since the wind speed at a given location varies with time, the use of a larger blade may lower the minimum wind speed at which a turbine can be profitably operated, allowing turbines at a given site to be generating power a larger fraction of the time. This can result in a significant reduction in the overall cost of energy from wind turbines.
For instance, some of the first wind turbines were constructed of wood and canvas sails because of their affordability and easy construction. However, wood and canvas materials require a lot of maintenance over their service life. Also, their shape was associated with a low aerodynamic efficiency creating a relatively high tried for the force they were able to capture. For these reasons, wind turbine blades were replaced with a solid airfoils structures.
Other older style wind turbines were designed with relatively heavy steel components in their blades (such as steel girders, cross bars, and ribs), which produced a higher rotational inertia. While improved in aerodynamic efficiency, structural durability and maintenance, the speed rotations in heavy steel blades required governance by an alternating current frequency of the power source to buffer the changes in rotation speed to thus, make power output more stable. Furthermore, the weight of steel becomes economically prohibitive in designing longer blades capable of rotating in large arcs within the low-speed wind areas.
Subsequent methods of forming wind blade airfoils involved using aircraft construction techniques. These techniques included using heavy balsa wood laid across the main metal or wood bar of a blade running down the length of the blade. Many of these types of blades used a set of ribs providing chord wise support and maintaining airfoil shape. Skins of sheet metal were riveted to the rigid ribs therein to provide the aerodynamic surface. While lighter than primarily steel blades, these designs still suffer from the shortcomings associated with the economics of weight per blade unit length of components.
Currently, wind turbine blade fabrication mimics the same techniques used in boat building and surfboard construction. Some current conventional wind turbine blades are manufactured at a length approximately 100 to 150 feet long. Materials of choice are commonly fiberglass with epoxy resin forming airfoils using wet layup techniques. The blades are fabricated in large costly “clamshell” molds where skins and heavy glass balsa panel cores are laid up manually. Such solid fiberglass structures are relatively heavy for a 31 meter blade (approximately 12,000 pounds) and require expensive tooling for full-scale heated molds.
Other more sophisticated techniques include a turbine with blades that can be twisted in response to variable torque forces. A device of this type can be seen in U.S. Pat. No. 5,284,419 to Lutz.
It can be seen therefore, that a need exists in the art for a wind turbine blade made from sturdy construction capable of withstanding sudden wind loads yet, is lightweight, economical, and materially efficient for production in longer lengths capable of generating power in low wind speed areas. Additionally, a need exists for such a blade that can be readily disassembled for shipment in standard transportation containers and readily assembled on site.